Marginal-Marine Environments

Deltas, Beach and Barrier Island systems,

Marginal Marine

• Transitional

• Where land meets the sea

• Dominated by rivers (fluvial), wave, and tidal processes.

• High-energy – waves and currents

• Quiet water – lagoonal, and estuarine

Deltas – Deltaic system

• Alluvial Delta

• Subaerial, subaqueous

• Sediment in a standing body of water

Ancient Delta Deposits

• Found in all ages of stratigraphic sequences

• Important petroleum and natural gas deposits, coal, and other mineral resources, uranium is one.

• Excellent reservoir rocks, cap rocks and traps in this sequence of rocks, stratigraphic traps

Modern deltas

• Table 9.1

• Largest delta plain area = Ganges –Bramaputra

• Large water discharge = G-P, Irrawaddy, Mississippi

Classification of Deltas

Delta Classification 2Figure 9.3

Cross-Section of a Delta

progradation

Delta formation

Cross-section

Gilbert-type deltas, in your book, Figure 9.4

Components of a Delta

Figure 9.12

Sediment Characteristics of Deltas

• Deltaic plain – subaerial component

• Subaqueous Deltaic plaine – Delta front– prodelta

Closer view of the cross-section

Flow and Discharge

• Jet – discharge of sediment• Homopycnal flow – equal density water of river

entering basin of equal density water, rapid mixing and abrupt deposition of sediment.

• Hyperpycnal flow – river water higher density than basin water, flows beneath the basin water, vertically oriented plane-jet flow, forms turbidites on gentle slope of delta.

• Hypopycnal flow – river water is less dense than the basin water, river flow into marine or saline lake flow on top of basin as a horizontal plane-jet,

Bird-foot Delta

History of the Mississippi Delta

River-Dominated

Delta

Wave-dominated Delta

Tidal-dominated Delta

Fan Delta

Figure 9.11

Delta Cycles

• Progradation of delta seaward – construction phase– Coarsening-upward sequence

• Fine prodelta muds overlain by delta front silt and sand

• Distributary-mouth sands on top of these• Marsh and fluvial deposits

• Transgression of sea– Destruction phase

River – Dominated

Strata

Wave-dominated delta Strata

Tide-dominated delta strata

Ancient Delta system -

Mississippian

What tectonic event would these deltas be associated with of the Appalachian Mountains?

What do you know of the sea?

• Ocean currents?– Rotation?– Hurricanes rotate in what direction?

• What about tides?– How many low tides per day?– Spring tide?– Neap tide?

Surface Currents

Spring Tide and Neap Tide

Beach and Barrier Island Systems

• Beach is the dominant marginal marine deposit

• What is the dominant Wentworth’s clast size found on the beach?

Depositional Settings

• Tidal ranges– Microtidal = 0 – 2 m tidal range, barrier

islands– Mesotidal = 2 – 4 m tidal range– Macrotidal = > 4 m tidal range

Beach to Barrier Island

• Single Beach – attached to the mainland• Strand Plain – broad beach-ridge system,

multiple parallel beach ridges and parallel swales, lack well-developed lagoons or marshes– Chenier Plain – sandy ridges elongated along

the coast and separated by coastal mudflat

• Barrier Island – separated wholly or partly form mainland by a lagoon or marsh

Beach

Littoral zone

Breakers

Longshore Current

Rip Current

Prograding Beach

Figure 9.25

Barrier Islands and

Lagoon

Barrier IslandFigure 9.23

Characteristics

• Beach deposits– Fine to medium Sand– Heavy-mineral laminae

• Backshore– Eolian sand deposits– Storm-wave deposits

• Shoreface deposits– Upper shoreface – bidirectional cross-bedding sets, Skolithos

burrows, – Middle shoreface – fine to medium sand, shell material,

landward and seaward dipping cross-beds, Skolithos and ophiomorpha

– Lower shoreface – fine to very fine sand, intercalated layers of silt and mud, small scale cross-stratification, horizontal laminations, hummocky cross-stratification

Deposits

• Back-barrier– Washover deposits – sandy deposits in the mud-rich

lagoon– Tidal-channel deposits – sand and lag deposits, fluvial

marine deposit, cross-bedding– Tidal delta deposits – sandy deposits, parabolic

shape in cross-section– Tidal Flat deposits – sand lenses and mud deposit– Lagoonal – fine grained muds, organic rich muds– Marsh – sandy to silts to muds, peat deposits,

bioturbated a lot.

Barrier Islands

Figure 9.26

Galveston Island

Delaware Island

Barrier Island

Sequence

Back-Barrier

Figure 9.27

Pennsylvanian Lagoon

Barrier Island and Lagoon -

Estuary

Estuary